The present invention relates to a protection circuit for protecting a semiconductor laser device from a surge, which device is to be used in an optical pickup for recording and reproducing information on and from an optical disk.
Recently, recording and reproducing devices of personal computers, and music CD players have been used in large quantities, and ever-increasing demand for optical pickups using semiconductor laser devices is presented. Under such circumstances, it has become essential to take countermeasures against static electricity of semiconductor laser devices in the production process and practical use of optical pickups and optical disc devices.
Laser diode chips (hereinafter referred to as “LD chips”) that are built in semiconductor laser devices are subject to breakdowns due to surges caused by static electricity because sizes of electrodes are as small as 200-300 square micrometers.
As a protection circuit that protects an LD chip from a surge, a circuit as shown in FIG. 8 is known, in which a capacitor C and a coil L are combined (for example, see “SEMICONDUCTOR DATA BOOK”, 1998, page 18, Sharp). In the protection circuit shown in FIG. 8, a capacitor C having a capacitance of 0.3-1.5 μF and a coil L having an inductance of 10-100 μH are usually used.
Further, as another circuit constitution, a protection circuit as shown in FIG. 9 is also proposed in which a coil L is inserted in series with an LD chip 11 on its anode side, and a capacitor C is connected in parallel with the LD chip 11 (JP-A-60-38894).
When using the protection circuit shown in FIG. 8 or 9, a dielectric strength of about 2 kV in terms of electrostatic breakdown voltage can be obtained.
The electrostatic breakdown voltage herein means a physical property measured using an electrostatic test circuit (in conformity with EIAJ-4701A) shown in FIG. 6. A method of testing an electrostatic breakdown voltage using the electrostatic test circuit will be described with reference to FIG. 7.
First, a PH (optical output power)—Iop (I-L) characteristic of an LD chip is measured (an I-L curve with a legend “BEFORE TEST” in FIG. 7). Next, with the voltage of an internal power supply 31 of the electrostatic circuit shown in FIG. 6 set for “V” (kV), a switch 32 is connected to an X terminal so that a capacitor 34 having a capacitance of 200 pF is charged through a charging resistor 33. After completion of the charging, the switch 32 is connected to a Y terminal so that an electrical current flows through the LD chip via a circuit board 12 on which a semiconductor laser device including the LD chip is mounted. The internal resistance of the electrostatic test circuit is set for 0 Ω.
The above process is repeated five times at intervals of one second and then the I-L characteristic of the LD chip is measured again. If a current value (Iop) passing the LD chip at a prescribed optical output power increases by 20% or less after the test, compared with that before the test, the electrostatic breakdown voltage is judged to be “V” or higher (for example, the I-L curve with a legend “NON-DEFECTIVE AFTER TEST” in FIG. 7).
The similar experiment is further repeated while increasing the voltage of the internal power supply 31. When an Iop value at a prescribed optical output power increases by at least 20% compared with that before the test, the voltage immediately before the increase is judged to be an electrostatic breakdown voltage. When a voltage of more than the electrostatic breakdown voltage is applied through the circuit board 12, the semiconductor laser device deteriorates, resulting in an optical output power (PH) of at most 5 mW in many cases (for example, the I-L curve with a legend “DETERIORATION” in FIG. 7).
Incidentally, when using the conventional protection circuit shown in FIG. 8 or 9, the electrostatic breakdown voltage is about 2 kV as described above. From this, it is concluded that the protection circuit achieves a considerable effect compared with the case where there is no protection circuit (in which case the electrostatic breakdown voltage is less than 100 V).
However, with the recent mass production of optical pickups, countermeasures against static electricity in the production process have been simplified. As a result, a higher electrostatic breakdown voltage (for example, more than 2 kV) has been required.